A freed hostage speaks to relatives from a hospital window in Moscow. (AFP)

Moscow, Oct. 27 (Reuters): All but two of the 117 hostages so far confirmed dead in the Moscow theatre siege died of gas poisoning, the city’s top doctor said.

Andrei Seltsovsky, chairman of the health committee of the city of Moscow, said only one of the around 800 hostages had died from gunshot wounds when elite troops stormed the musical theatre early on Saturday.

Asked what the others had died from, he said: “From the effects of the gas exposure.”

One man had been shot dead during the operation to free the captives. The second to die of gunshot wounds was a woman shot while trying to escape when the theatre was seized by around 50 Chechen guerrillas on Wednesday night.

Seltsovsky told a news conference that 646 of the freed hostages were still in hospital, of whom 150 were in intensive care and 45 were “in a grave condition”.

The unidentified chemical was so powerful that the Chechen suicide fighters had no time to detonate the explosives strapped to their waists.

Sergei, 36, who declined to give his family name, said after he was released from hospital that the gas had smelled slightly bitter. Chemical warfare experts say nerve gas often smells of bitter almonds.

London-based security expert Michael Yardley said he believed the gas used was BZ, a colourless, odourless incapacitant with hallucinogenic properties, first used by the US in Vietnam.

He said the symptoms displayed by the hostages in Moscow — inability to walk, memory loss, fainting, heartbeat irregularities, sickness — all pointed to BZ. According to the US army, the side effects last 60 hours, Yardley said.

“The Russians wouldn’t want a big shout about it because it (BZ) is just the sort of stuff they are not supposed to have,” he said. “It’s not specifically banned, but... it is in a sort of grey area.”

Film taken after the special forces stormed the theatre to free more than 750 hostages showed a woman slumped back on a chair with her mouth wide open and a bag of explosives tied to her front.

“A panic went up among us and people were screaming ‘Gas! gas!’ and, yes, there was shooting,” theatre director Georgy Vasilyev, one of the hostages, said.

“But then everyone fell down quickly. And then I was told by one woman while we were in hospital together, but who didn’t fall asleep immediately because she covered her mouth and nose, that it was very strange to look at everyone.

“You see, when the shooting began, they (the rebels) told us to lean forward in the theatre seats and cover our heads behind the seats. But then everyone fell asleep. And they (the rebels) were sitting there with their heads thrown back and their mouths wide open.”

One hostage told Interfax news agency that he saw the guerrillas convulse and slump because of gas. “After the first shots at the hostages, gas came in. I saw how a terrorist sitting at the scene jumped up and tried to get a respirator. I saw how he convulsed and tried to put the mask to his face and then fell,” the unidentified witness said.

In Moscow, distraught relatives begged for information on loved ones, and police checked cars, passengers and luggage to prevent a feared repeat attack.

Police said they could not immediately confirm a report that three Chechens had been charged with involvement in preparations for the hostage-taking.

Local television said police detained a Chechen woman who had been in hospital on suspicion of involvement in the attack.

The heightened security meant that many Russians could not get in to see relatives being treated in hospitals.

President Vladimir Putin asked for forgiveness from the relatives of the dead.